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Beomaster 8000 right channel went up in smoke, what to do next?

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BeoMaster4
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BeoMaster4 posted on Sun, Oct 21 2012 9:34 PM

Hey guys,

My first post here at beoworld. My parents had an old Beomaster 8000 set in the garage (beomaster, beocord, beogram and beovox ms150) which has been unused for 8 years or so. We brought the set back to life two days ago. All the devices have their problems, but the beomaster was working ok (apart for half of the buttons on it). We have used the beomaster for about two days now and some time at full volume, all was well.

Today we were just watching TV and had the volume set about half way when suddenly it made a loud pop on the right channel and smoke came out on the right side of the heat sink.

I disassembled the unit right away to look for anything obvious. No visible damage can be seen, but a very distinct burned smell comes from the number 3 column of metal plates on the heatsink (numbering the columns from left to right, so 3 and 4 being the right side). I have no idea what these things do, but apparently they get hot...

I hope you can advise me on what to do next. I guess these machines have common problems. I already read about the right channel running hot. It's strange I can't see any damage, while it produced a fair bit of smoke and smell.

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Søren Mexico
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Where do you live, its easier to help if we know at least your country

Collecting Vintage B&O is not a hobby, its a lifestyle.

BeoMaster4
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I live in The Netherlands

Leslie
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Leslie replied on Sun, Oct 21 2012 10:04 PM

We have some well experienced and respected Dutch members such as Blackrix, Ipaul and Beobuddy who can help you. Try the Dutch forum en ik weet zeker dat je antwoord zult krijgen...

Brengen & Ophalen

Søren Mexico
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Leslie:

We have some well experienced and respected Dutch members such as Blackrix, Ipaul and Beobuddy who can help you. Try the Dutch forum en ik weet zeker dat je antwoord zult krijgen...

Hey hey, dont run away to the Dutch forum, we like to know what happens, and if we have to use the Google translator it gets kind of problematisBig Smile

 

Collecting Vintage B&O is not a hobby, its a lifestyle.

chartz
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chartz replied on Mon, Oct 22 2012 7:40 AM

Hi,

Sometimes when a transistor gives up the ghost, there is a tiny crack on its surface and you will really have to look with a magnifying glass to see it!

Probably bad caps and idle (quiescent) current trimmer caused that...

Jacques

Orava
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Orava replied on Mon, Oct 22 2012 8:26 AM

And that crck might be towards heatsink, invisible to front. Power resistors (0,18ohm) has possibly burnt too.

 blah-blah and photographs as needed

ipaul
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ipaul replied on Mon, Oct 22 2012 9:44 AM

...and obviously all depends how experienced you are with electronics :)

Probably indeed (unfortunately) some short in a power output stage which causes a smoking resistor :(, which will than need some soldering, replacing and probably adjusting.

Maybe a magnifying glass will help, sometimes it's just hard to see.

Nice set though, too good for a garage :), hope it was dry and not cold there, as that may cause trimmers to go bad etc...

Dillen
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Dillen replied on Mon, Oct 22 2012 12:25 PM

Typical case of an open idle current trimmer.
The output stage heats up and makes whats called a thermal runaway with the output stage pulling maximum from the power supply.
The power supply will deliver a good 700 Watts so the output stage, however beefy, becomes the weakest point.
It all ends in a blown output stage, most often dragging driver transistors etc. with it in the fall.

Replace all components in the output stage of the affected channel, preferably all components on the board plus those on the cooling fin
and replace trimmers and capacitors in the other channel to prevent it from happening here too.

This is a job for a skilled tech guy but worth it for an otherwise good Beomaster 8000.

Martin

BeoMaster4
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Thanks for all the advice so far.

The Beomaster also has some other problems, the tp2 channel is not working and pretty much all the controls on the controls panel are non responsive. (bass, treble, filters etc). So... maybe it is not worth rescuing anymore?

I do have a fair bit of experience soldering electronics, so that shouldn't be a problem. On the other hand, I don't have a lot of knowledge about stuff like this.

Martin, I also just ordered a cap kit for my Beogram from you. Do you also sell components to fix up the beomaster? Or do I have to start disassembling the output stage and source new components locally?

Is there a guide or something to the PCB's and it's components on this machine?

Dillen
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Dillen replied on Mon, Oct 22 2012 1:00 PM

Yes, there are some kits etc. but, honestly - and don't get me wrong, this is not a job for you unless you are more than fairly
skilled and educated with electronics in general and powerful DC coupled soft-clipping complimentary output stages in particular.
Several, and even some authorized, repairshops would reject this repair simply because they don't have the training and experience needed
and the Beomaster 8000 is a rather complex beast and definitely not a machine for beginners.
If you don't know where to locate and access the components mentioned above, I dare say you don't have the skills to perform this repair
and I suggest you bring it to someone with a good experience in these machines.
A good repairer will not be that costly and he can fix the other issues too which sounds like caused by lack of use etc.

Martin

Leslie
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Leslie replied on Mon, Oct 22 2012 7:54 PM

Dillen:
If you don't know where to locate and access the components mentioned above, I dare say you don't have the skills to perform this repair
and I suggest you bring it to someone with a good experience in these machines.

And that's why I recommended these Dutch techies who might be able to help you in your own language, simple as that!

Brengen & Ophalen

BeoMaster4
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Hey guys,

Thanks for your advice. But I'd like to try to fix this myself if at all possible. Part of the fun of something like this is fixing something yourself rather than paying someone to do it for you. I've put together a fuel injection system for my car recently, which was a lot more hairy than having to solder a board like this together. I understand you advise not to do it, but you only challenged me to do it myself :)

I removed the output stage today:

I was hoping that removing the board would reveal anything visually obvious, but it didn't. Only C212 has a body that has slightly cracked on the side. Besides that there is a very distinct burnt smell from one of the left three power transistors on the cooling fins.

So, what step should I take now? I was thinking I may want to remove the left power transistors and replace them. After that also rebuild the boards (both left and right) as described here: http://beolover.blogspot.nl/2012/08/beomaster-8000-left-output-stage-rebuild.html

Do you think this would be a good idea? Or do you suggest a different approach?

 

Dillen
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Dillen replied on Tue, Oct 23 2012 3:25 PM

The blown transistors have blown because the others have shorted.
Replace all components on the cooling fin and the board.
Adjust idle current and DC offset to factory specs and it will be playing again.

Martin

Orava
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Orava replied on Tue, Oct 23 2012 3:29 PM

Do B&O use matched pairs?

 blah-blah and photographs as needed

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